SQLite

  w.xExprCallback = exprIdxCover;
  w.u.pIdxCover = &xcov;
  sqlite3WalkExpr(&w, pExpr);
  return !w.eCode;
}



/* Structure used to pass information throught the Walker in order to

** implement sqlite3ReferencesSrcList().

*/
struct RefSrcList {
  sqlite3 *db;         /* Database connection used for sqlite3DbRealloc() */
  SrcList *pRef;       /* Looking for references to these tables */

  int nExclude;        /* Number of tables to exclude from the search */
  int *aiExclude;      /* Cursor IDs for tables to exclude from the search */
};

/*
** Walker SELECT callbacks for sqlite3ReferencesSrcList().
**
** When entering a new subquery on the pExpr argument, add all FROM clause

** entries for that subquery to the exclude list.
**
** When leaving the subquery, remove those entries from the exclude list.
*/
static int selectRefEnter(Walker *pWalker, Select *pSelect){
  struct RefSrcList *p = pWalker->u.pRefSrcList;
  SrcList *pSrc = pSelect->pSrc;
  int i, j, *piNew;
  if( pSrc->nSrc==0 ) return WRC_Continue;
  j = p->nExclude;
  p->nExclude += pSrc->nSrc;
  piNew = sqlite3DbRealloc(p->db, p->aiExclude, p->nExclude*sizeof(int));
  if( piNew==0 ){
    p->nExclude = 0;
    return WRC_Abort;
  }else{
    p->aiExclude = piNew;
  }
  for(i=0; i<pSrc->nSrc; i++, j++){
     p->aiExclude[j] = pSrc->a[i].iCursor;
  }
  return WRC_Continue;
}
static void selectRefLeave(Walker *pWalker, Select *pSelect){
  struct RefSrcList *p = pWalker->u.pRefSrcList;
  SrcList *pSrc = pSelect->pSrc;
  if( p->nExclude ){
    assert( p->nExclude>=pSrc->nSrc );
    p->nExclude -= pSrc->nSrc;
  }
}

/* This is the Walker EXPR callback for sqlite3ReferencesSrcList().
** 
** Set the 0x01 bit of pWalker->eCode if there is a reference to any
** of the tables shown in RefSrcList.pRef.
**
** Set the 0x02 bit of pWalker->eCode if there is a reference to a
** table is in neither RefSrcList.pRef nor RefSrcList.aiExclude.
*/
static int exprRefToSrcList(Walker *pWalker, Expr *pExpr){






  if( pExpr->op==TK_COLUMN
   || pExpr->op==TK_AGG_COLUMN
  ){
    int i;
    struct RefSrcList *p = pWalker->u.pRefSrcList;
    SrcList *pSrc = p->pRef;
    int nSrc = pSrc ? pSrc->nSrc : 0;
    for(i=0; i<nSrc; i++){
      if( pExpr->iTable==pSrc->a[i].iCursor ){
        pWalker->eCode |= 1;
        return WRC_Continue;
      }


    }
    for(i=0; i<p->nExclude && p->aiExclude[i]!=pExpr->iTable; i++){}



    if( i>=p->nExclude ){
      pWalker->eCode |= 2;
    }
  }
  return WRC_Continue;
}

/*
** Check to see if pExpr references any tables in pSrcList.
** Possible return values:
**
**    1         pExpr does references a table in pSrcList.
**
**    0         pExpr references some table that is not defined in either
**              pSrcList or in subqueries of pExpr itself.
**
**   -1         pExpr only references no tables at all, or it only
**              references tables defined in subqueries of pExpr itself.
**
** As currently used, pExpr is always an aggregate function call.  That
** fact is exploited for efficiency.
*/
int sqlite3ReferencesSrcList(Parse *pParse, Expr *pExpr, SrcList *pSrcList){
  Walker w;
  struct RefSrcList x;

  memset(&w, 0, sizeof(w));
  memset(&x, 0, sizeof(x));
  w.xExprCallback = exprRefToSrcList;
  w.xSelectCallback = selectRefEnter;
  w.xSelectCallback2 = selectRefLeave;
  w.u.pRefSrcList = &x;
  x.db = pParse->db;
  x.pRef = pSrcList;



  assert( pExpr->op==TK_AGG_FUNCTION );
  assert( ExprUseXList(pExpr) );
  sqlite3WalkExprList(&w, pExpr->x.pList);
#ifndef SQLITE_OMIT_WINDOWFUNC
  if( ExprHasProperty(pExpr, EP_WinFunc) ){
    sqlite3WalkExpr(&w, pExpr->y.pWin->pFilter);
  }
#endif
  sqlite3DbFree(pParse->db, x.aiExclude);
  if( w.eCode & 0x01 ){
    return 1;
  }else if( w.eCode ){
    return 0;
  }else{
    return -1;
  }
}

/*
** This is a Walker expression node callback.
**
** For Expr nodes that contain pAggInfo pointers, make sure the AggInfo
** object that is referenced does not refer directly to the Expr.  If

          sqlite3WalkExprList(pWalker, pWin->pOrderBy);
          sqlite3WalkExpr(pWalker, pWin->pFilter);
          sqlite3WindowLink(pSel, pWin);
          pNC->ncFlags |= NC_HasWin;
        }else
#endif /* SQLITE_OMIT_WINDOWFUNC */
        {
          NameContext *pNC2;          /* For looping up thru outer contexts */
          pExpr->op = TK_AGG_FUNCTION;
          pExpr->op2 = 0;
#ifndef SQLITE_OMIT_WINDOWFUNC
          if( ExprHasProperty(pExpr, EP_WinFunc) ){
            sqlite3WalkExpr(pWalker, pExpr->y.pWin->pFilter);
          }
#endif
          pNC2 = pNC;
          while( pNC2 
              && sqlite3ReferencesSrcList(pParse, pExpr, pNC2->pSrcList)==0
          ){
            pExpr->op2++;
            pNC2 = pNC2->pNext;
          }
          assert( pDef!=0 || IN_RENAME_OBJECT );
          if( pNC2 && pDef ){
            assert( SQLITE_FUNC_MINMAX==NC_MinMaxAgg );
            assert( SQLITE_FUNC_ANYORDER==NC_OrderAgg );

  int walkerDepth;                          /* Number of subqueries */
  u16 eCode;                                /* A small processing code */
  union {                                   /* Extra data for callback */
    NameContext *pNC;                         /* Naming context */
    int n;                                    /* A counter */
    int iCur;                                 /* A cursor number */
    SrcList *pSrcList;                        /* FROM clause */

    struct CCurHint *pCCurHint;               /* Used by codeCursorHint() */
    struct RefSrcList *pRefSrcList;           /* sqlite3ReferencesSrcList() */
    int *aiCol;                               /* array of column indexes */
    struct IdxCover *pIdxCover;               /* Check for index coverage */
    struct IdxExprTrans *pIdxTrans;           /* Convert idxed expr to column */
    ExprList *pGroupBy;                       /* GROUP BY clause */
    Select *pSelect;                          /* HAVING to WHERE clause ctx */
    struct WindowRewrite *pRewrite;           /* Window rewrite context */
    struct WhereConst *pConst;                /* WHERE clause constants */

int sqlite3ExprListCompare(const ExprList*,const ExprList*, int);
int sqlite3ExprImpliesExpr(const Parse*,const Expr*,const Expr*, int);
int sqlite3ExprImpliesNonNullRow(Expr*,int);
void sqlite3AggInfoPersistWalkerInit(Walker*,Parse*);
void sqlite3ExprAnalyzeAggregates(NameContext*, Expr*);
void sqlite3ExprAnalyzeAggList(NameContext*,ExprList*);
int sqlite3ExprCoveredByIndex(Expr*, int iCur, Index *pIdx);
int sqlite3ReferencesSrcList(Parse*, Expr*, SrcList*);
Vdbe *sqlite3GetVdbe(Parse*);
#ifndef SQLITE_UNTESTABLE
void sqlite3PrngSaveState(void);
void sqlite3PrngRestoreState(void);
#endif
void sqlite3RollbackAll(sqlite3*,int);
void sqlite3CodeVerifySchema(Parse*, int);